Industrial lubricants are often required to provide good friction reducing properties under thin-film or boundary conditions. Flat bearings, such as slideways, guides and ways used on forging and stamping presses; as crosshead guides of certain compressors, diesel and steam engines; and on metalworking machines such as lathes, grinders, planers, shapers and milling machines, for example, can present special problems. At low speeds and under heavy loads, the lubricant tends to be wiped off so that boundary lubrication prevails. Machine tools for precision machining in particular generally require slides and ways to operate under boundary conditions at all times. A phenomenon known as "stick slip" can be encountered in the motion of slides and ways if the static coefficient of friction of the lubricant is greater than the dynamic coefficient, requiring more force to start the sliding motion from rest than that required to maintain the motion after it has started.
Another phenomenon known as "float" can be encountered in the motion of slides and ways with low loads and high traverse speeds if the oil viscosity is high, resulting in lifting the slide from the way which, with variations in speed or load, can vary the lubricant film thickness enough to produce wavy surfaces on parts being machined, or cause parts to be made offsize.
Rotating bearings, such as plain bearings or anti-friction (i.e., rolling) bearings, lead screws and nuts, gears, hydraulic systems, and pneumatic devices also often encounter low speed and heavy load conditions, particularly in an industrial setting where these are often components found in machine tools and other heavy industrial machinery, although low speed/heavy load conditions sometimes are found in non-industrial settings as well, such as in components found in land vehicles, ships and aircraft. Lead screws and nuts are often used, for example, to control the flaps on the wings of medium to large airplanes.
Improved friction reduction and reduced stick slip under boundary conditions has generally required employing friction reducing and extreme pressure/antiwear additives in the lubricant to compensate for the corresponding deficiencies in the lubricant oil.
Many friction-modifying or extreme pressure/antiwear additives, however, often have problems such as toxicity to humans, unpleasant smell such as from the release of sulfur gases from extreme pressure/antiwear agents containing sulfur, and/or the addition of an opaque color making equipment maintenance difficult and messy, so that it is advantageous to obtain the desired friction-modification and extreme pressure/antiwear properties without such additives. With the invention presented in this application, the inventors have found that metal overbased unsaturated linear hydrocarbon carboxylates are able to achieve the desired friction reducing and extreme pressure/antiwear protection without additional friction-modifying or extreme pressure/antiwear additives.
In addition, friction-modifying and extreme pressure/antiwear additives may be advantageously added to the metal overbased carboxylates used in the present invention to achieve even greater friction-modifying and extreme pressure/antiwear properties.
The terms "overbased", "superbased", and "hyperbased", are terms of art which are generic to well known classes of metal-containing materials which for the last several decades have been employed as detergents and/or dispersants in lubricating oil compositions. These overbased materials which have also been referred to as "complexes", "metal complexes", "high-metal containing salts", and the like, are characterized by a metal content in excess of that which would be present according to the stoichiometry of the metal and the particular organic compound reacted with the metal, e.g., a carboxylic or sulfonic acid.
Newtonian overbased materials and non-Newtonian colloidal disperse systems comprising solid metal-containing colloidal particles pre-dispersed in a disperse medium of at least one inert organic liquid and a third component selected from the class consisting of organic compounds which are substantially insoluble in said disperse medium are known. See, for example, U.S. Pat. Nos. 3,492,231; and 4,230,586.
Carboxylic acid derivatives made from high molecular weight carboxylic acid acylating agents and amino compounds and their use in oil-based lubricants are well known. See, for example, U.S. Pat. Nos. 3,216,936; 3,219,666; 3,502,677; and 3,708,522.
Metal working lubricants containing a lubricating oil and a basic metal salt or borated complex thereof, including overbased carboxylates, are described in U.S. Pat. Nos. 4,659,488; 4,505,830; and 3,813,337.